The present invention relates to apparatus and method for determining the conductive state of a control turn-off semiconductor and to such method and apparatus as applied to an electric power converter employing series connected control turn-off semiconductors between direct current (dc) buses to prevent the rendering a one of the semiconductors in the series connection conductive before the other has become non-conductive.
There are a number of situations in which the conductive state of a semiconductor device is required to be known. It may, for example, be required to know the state of conduction for alarm or total shutdown purposes of a system. More commonly, in many power converters, there are included two semiconductors connected in a series arrangement in what is commonly referred to as a "leg" between the buses of a dc source. These semiconductors serve to control the electric power supplied to a load. A common converter of this type is a three-phase converter having three legs connected in mutual parallel between positive and negative dc buses. The semiconductors of the legs are rendered conductive in a predetermined order or sequence in order to control the electrical power delivered from the dc buses to the load. If both semiconductors of any one leg become simultaneously conductive, it is apparent that there will exist between the two dc buses a short circuit which, if allowed to continue may have disastrous results to the load, the power source and/or to the semiconductors themselves. If the semiconductor devices are of the type which require signals to a control electrode in order to render the device selectively conductive and non-conductive, the problem becomes more acute since devices of this nature, as presently known, are very limited in the amount of current which they can interrupt or turn-off. Devices of this nature common in today's discipline are referred to as gate turn-off thyristors and power transistors. Collectively, such devices are referred to in this application as "control turn-off semiconductors".
There are several methods and apparatus for determining the conductive capabilities of a control turn-off semiconductor. For example, the recently invented scheme described in the aforementioned patent application Ser. No. 805,645, which is cross referenced to this application, employs current transformers and determines the general operational capability of the control turn-off semiconductor prior to application of full power to the device. This scheme is not, however, well suited to the detection of the conductive state of the device during full operation.
One method of attempting to avoid the earlier referenced dc short circuit between the buses is to simply provide a delay between the gate pulses turning one of the devices of a leg off and the gate pulses turning the other device on. So long as the delay is longer than the time it takes to turn the semiconductor off, a short is usually prevented. This system has the disadvantage in that it is not positive in its action; i.e., there is no positive indication that the first device has actually turned off. Additionally, it is not well adapted to high performance systems since the delay must be sufficient to permit the first semiconductor to turn off under all operating circumstances.
Another system is what is referred to as the anode sensing method. This system monitors current direction and the semiconductor anode to cathode voltage. If the current polarity is positive, then the turn-off of that semiconductor will be indicated by the appearance of a positive voltage from the anode to cathode. Thus, by delaying the gating of the second semiconductor of the leg until the voltage appears across the first, a short circuit of the dc source can be prevented. This system, however, does not work if the current is negative; that is, being carried by the diode which is normally connected in antiparallel in these types of systems. In this case, because the diode is conducting, the voltage sensed will stay very small. If the second control turnoff device is gated on before the first is actually turned off, or when the first has failed in a shorted mode then a short circuit will exist.